1. Field of the Invention
The present invention generally relates to the formation of insulating layers on substrates, and more particularly to a carbon-graded layer that improves the adhesion between low-k dielectrics and substrates.
2. Description of the Related Art
Insulating layers are used to separate conductor and semiconductor layers within semiconductor devices. Recently, low-k dielectrics have become popular because they create less capacitance between and around the conductors and are more easily applied than conventional silicon oxide dielectrics, which have higher dielectric constants. The recent progress in low-k dielectrics using CVD techniques offers more affordable and attractive dielectric options to the advanced interconnect technologies. For example, by employing CVD low-k with a dielectric constant of 2.7 at the wiring level, the total capacitance and RC delay can be significantly reduced.
However, one disadvantage of using low-k dielectrics is the poor adhesion between the low-k dielectrics and the underlying substrate. Conventional methods typically form low-k dielectric films through either spin on processes or Plasma Enhanced Chemical Vapor Deposition (PECVD) of organosilane gases, such as amorphous hydrogenated carbon doped oxide (a-SiCO;H), etc. However, such gases have poor chemical adhesion to substrates, such as silicon dioxide, silicon nitride, silicon carbide, silicon, tungsten, aluminum, and copper substrates. Because of this low structural adhesion, low-k dielectric layer are often delaminated from the underlying substrate, which leads a failure of interconnect processes. One of the conventional methods to improve the adhesion between low-k dielectric layers and underlying substrates is the use of an adhesion promoter. However, an adhesion promoter is often used for spin on dielectric (SOD) low-k rather than PECVD processes, which requires the use of a precursor such as methylsilane (1MS) trimethylsilane (3MS), tetramethylsilane (4MS), tetramethylcyclotetrasiloxane (TMCTS), and orthomethylcyclotetrasiloxane (OMCTS). However, such low-k films, in general, have a hydrophobic surface with high wetting angles with water. This causes these films to have a very poor adhesion with substrate layers.
Therefore, there is a need for a structure and method that improves the adhesion between low-k dielectrics and their corresponding substrates.
It is, therefore, an object of the present invention to provide a structure and method for an insulator layer having carbon-graded layers above a substrate, wherein the concentration of carbon increases in each successive carbon-graded layer above the substrate. The insulator comprises a low-k dielectric having a dielectric constant less than 3.3 (preferably 2.7). The carbon-graded layer increases adhesion between the substrate and the insulator and between the insulator and the conductor layer. The process may also include stabilization steps between the carbon-graded layer deposition steps. More specifically, the carbon-graded layers include a first layer adjacent the substrate having a carbon content between about 5% and 20%, a second layer above the first layer having a carbon content between about 10% and 30%, and a third layer above the second layer having a carbon content between about 20% and 40%.
The invention also includes a method of forming a semiconductor structure having an insulating layer comprising forming a substrate, depositing a layer containing a concentration of carbon on the substrate and repeating the depositing process forming successive carbon-graded layers over one another using increasing concentrations of carbon to form the insulator layer. More specifically, the depositing process comprises flowing a gas containing carbon element and a gas containing oxygen in a chamber to form a low-k dielectric having a dielectric constant less than 3.3. The invention increases a flow rate of the carbon containing gas and decreases a flow rate of the inert gas to increase the carbon concentration in each successive carbon-graded layer while the total gas flow of the carbon containing gas and inert gas flow is kept at a constant flow rate.
The invention overcomes the problem of poor adhesion between low-k dielectrics and substrates by forming a carbon-graded area between the low-k dielectric and the underlying substrate.